1. Field of the Invention
The present invention relates to a planar illumination device.
2. Description of Related Art
Non-emissive display devices such as LCD panels and logo display boards are widely used in many devices. External light sources are used to illuminate the non-emissive display devices. Light emitting diodes (LEDs) are the preferred light source in non-emissive display devices instead of cold cathode fluorescent lamps (CCFLs) because of its high brightness, long life-span, and a wide color gamut. Some research on LEDs are disclosed in a paper on Proceedings of the IEEE, Vol. 93, No. 10, entitled “Solid-State Lighting: Toward Superior Illumination,” published by Michael S. Shur et al., in October 2005, the disclosure of which is incorporated herein by reference.
A typical illumination device includes a plurality of LEDs as a light source. The LEDs are distributed over the illumination device and spaced apart from each other for better dissipation of heat generated by the LEDs. However, the LEDs emit light towards a target object along different directions, generating a shadow for the target object. These shadows superpose each other creating a multi-image phenomena effect which decreases the illumination efficiency of the illumination device.
Therefore an illumination device which can reduce the multi-image phenomena effect is desired to overcome the above described deficiencies.